This invention relates generally to a damper for use between corresponding parts as in a vehicle suspension application. In particular the invention deals with magneto-rheological fluid dampers and their ability to provide dampening characteristics in various power states.
Traditionally, a damper such as a shock absorber or a strut, is installed between corresponding parts where motion in a first part is to be damped in reference to a second part. A traditional damper consists of a body and a piston attached to a rod. The body has a smooth cylindrical interior surface and has an attachment feature at a closed end. The piston slides on the interior surface of the body, and the rod projects out an open end of the body. The open end where the rod projects is sealed around the rod, allowing only the rod to slide in and out of the body. An end of the rod outside the body has an attachment feature. Sealed inside the body is a liquid used as a damping fluid. Also, inside the damper body is a sliding piston that contains a volume of pressurized gas used to allow for fluid volume changes. In use, the attachment features of the rod and the closed end of the body are attached between parts to be damped such as between an automotive frame and a movable part of an automotive suspension. When the parts move, the damper strokes or changes length by the rod and piston moving within the body of the damper. When a force from the parts is applied to the damper, the force is transmitted to the damping fluid forcing the fluid through a restriction mechanism, thereby damping the parts. In magneto-rheological (MR) dampers, MR fluid (fluid containing iron particles) is used and the damping restriction mechanism is a magnetic flux established in the MR fluid. The magnetic flux is generated by electric coils with the level of damping generally proportional to the amount of electric current supplied to the coil. If no current is supplied to the damper coil, damping is at a minimum level. This presents obvious problems in that a current must be constantly supplied to the damper in order to provide a greater than minimum dampening effect by the MR damper.
Thus, what is desired is a MR damper that will provide a greater than minimum level of dampening when the power source is off.
The present invention is a MR damper that provides a higher than minimum level of damping when a power source to the MR damper is not supplying a control current. The damper of the present invention includes permanent magnets positioned to direct a magnetic flux across a MR fluid path causing a resistance to MR fluid flow. An electric coil cancels an affect of the permanent magnets when the control current is available to allow a control circuit more operating range. The permanent magnets allow for damping when no control current is available.
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the presently preferred embodiment. The drawings that accompany the detailed description are described as follows: